been made to spread an explosion. It is possible that under some conditions anthracite dust may ignite when suspended in the air, but there seems to be little danger of this under ordinary mining conditions. Moreover, in the long history of the Pennsylvania anthracite mines, some of which are very gaseous, there have never been any explosions so widespread as some that have taken place in bituminous coal mines.

Bituminous coal and lignite contain much larger proportions of volatile matter (matter that becomes gaseous on heating) than anthracite, and the larger proportions of volatile matter in these coals probably have much to do with the greater explosibility of their dusts. But some semibituminous coals, like those of the New River and Pocahontas fields in West Virginia, contain a smaller proportion of volatile matter than bituminous coals and yet give dusts that make intense explosions when strongly heated. So it seems that something besides the fineness of the dust or the composition of the coal fixes the ease with which the dust will spread an explosion. This other feature appears to be the structure of the coal; that is, the way in which the coal is bound together. Experiments being made at Pittsburg bear out this view. It is found that dusts which have about the same composition do not ignite with the same ease, and the resulting explosions do not have the same strength.

EFFECT OF STONE DUST.

It has been proved by experiments at Pittsburg and in Europe that if stone dust, clay, sand, or any other material that will not burn is mixed with coal dust, it keeps the coal dust from igniting easily. The more there is of such material in the mixture, the more difficult it is to set fire to the coal dust; and when the mixture contains more unburnable dust than coal dust, there seems to be little or no danger of the coal dust spreading an explosion. But the unburnable material must be fine and it must be well mixed with the coal dust to have its full effect. If the material is coarse or in lumps, like pieces of rock, and the dry coal dust is scattered around or over these lumps, or if, when there is much shale or clay dust on the floor, the pure dry coal dust has gathered on the ribs, timbers, or roof, any disturbance of the mine air may stir up enough coal dust to feed and extend an explosion.

EXPLOSIVENESS OF DUSTS.

As already stated, experiments at Pittsburg and elsewhere have shown that a mixture of air and the dust of bituminous coal or lignite may be explosive, even though the air contains no inflammable gas other than that from the dust. The records of mine explosions

confirm the results of these experiments, for there have been explosions in shallow mines in Iowa in which gas (methane) has never been found. Moreover, an explosion, once started in a dry, dusty mine, may travel through miles of entries and headings, even when there is not a trace of inflammable gas in the mine air. It is true, however, that the presence of even a small percentage of such gas greatly increases the chance of a dust explosion.

There have been many instances of explosions of coal dust in and about coal bins when the dust was raised into the air and set on fire. Similar explosions have happened in coal-crushing rooms. During the past year (1910) there were explosions of this kind at a plant in St. Louis, Mo., and at a cement plant in Colorado. In the latter explosion nine men were burned so badly by the flaming dust that they died.

It must not be thought that coal dust is the only kind of dust that will explode when no inflammable gas is present at the moment of explosion. Dusts of other substances containing carbon will do the same thing. Among such substances are flour, starch, and sugar. Many flour mills have been blown to pieces by dust explosions. The destruction of the big flour mills at Minneapolis in 1878 is a terrible example of ruin and loss of life caused by exploding flour dust. A gluten mill at Granite City, Ill., was seriously damaged by an explosion of gluten dust in 1910. Other mineral substances than coal have explosive dusts. There were several bad explosions of the dust of grahamite, a mineral containing carbon, at a mine in West Virginia. The dust of gilsonite, another carbon mineral, has exploded at mines in Utah. In December, 1910, a very violent explosion, apparently due to dust, occurred in an asphalt mine in Oklahoma.

CAUSES OF COAL-DUST EXPLOSIONS.

The more common means by which coal dust in the air may be ignited and start an explosion in a coal mine are as follows: The flame from a shot or the accidental ignition of an explosive in the open; an explosion of a small body of fire damp; and, more rarely, an electric arc or glowing wire or even the flame of an open lamp.

SHOTS OR BLASTS.

The long flame from a shot of black powder has been the commonest cause of coal-dust explosions in mines in this country. Blown-out shots have started many, and it is a common belief that the flame of black powder must come from a blown-out shot to start a dust explosion. This belief is not supported by the facts, for explosions have started from shots that were not blown out, nor even overcharged. It is certain, also, that if the air at the face contains even a small per

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centage of gas (methane), the flame from even a properly placed and charged shot of black powder may start a dust explosion. The danger is greatest when several shots in the same working place are fired at the same time. The dust made by a shot, and any gas distilled from the coal by the heat, may be set on fire by the flame of the next shot.

Dynamite is fully as dangerous as black powder. It is quicker than powder, but gives a long hot flame, and many explosions have been traced to its use. Miners have been known to put two explosives, like black powder and dynamite, in the same hole. Such a combination is more dangerous than either explosive used alone. If the quicker-burning explosive is placed at the outer end of the charge it may blow out the stemming; if it is placed at the bottom of the hole it may blow the slower explosive while still burning into the dust raised by the shot.

DANGER FROM TAMPING WITH COAL DUST.

The chance of a shot of some long-flame explosive like black powder or dynamite starting a dust explosion is fearfully increased if the shot is tamped with coal dust or fine coal. Sometimes miners wet the dust or machine cuttings used for stemming in the belief that they can thus keep it from being ignited. There is little ground for this belief. If the shot is blown out, its heat instantly drives off the water, and then sets fire to the cuttings. Experiments at Pittsburg have shown that if coal-dust stemming is wet it gives a flame only a little shorter than if dry. When 2.5 pounds of black powder and 2.6 pounds of coal-dust stemming were used in a cannon the flame with dry stemming was 64 feet long and with wet stemming 50 feet long. With heavier charges the flames would probably have been longer.

Sometimes miners use pieces of coal of nut size for stemming in the belief that they lessen the danger in this way. It is clear, however, that ramming or tamping such pieces will crush them, and that the shot will break the pieces still finer and perhaps ignite them.

FIRE DAMP.

Explosions of fire damp have ranked next to the use of long-flame explosives as a cause of dust explosions. Methane is given off more or less from nearly all coals, although most coal mines in the United States do not make enough methane to be called gaseous. A great many of them make a little methane, but so little that the return air currents carry under 0.5 per cent, a quantity too small to be detected with a good safety lamp. But if a mine is dry and dusty, and longflame explosives like black powder are used, the presence of a small percentage of methane greatly increases the chance of an extensive

dust explosion. Moreover, in those mines in which methane comes from the coal seam, the percentage in the mine air is usually larger at the working faces; and it is "at the face" that many explosions

start.

If methane comes from the floor or from the roof, it is likely to collect in the gob, or where there have been falls of roof, or in places where the air currents do not circulate. In a well-ventilated mine any body of fire damp is likely to be small, and if it is ignited there is little chance of the explosion spreading far unless dry coal dust is at hand. Therefore, if a mine makes any methane at all, great care should be taken to keep down the coal dust in all the workings.

ELECTRIC ARCS AND GLOWING WIRES.

That an electric arc or a glowing wire could ignite coal dust in the air was not thought possible a few years ago, but since then several mine explosions have been laid to these causes. Of course it must take a combination of unusual conditions to start an explosion in this way, but such a combination may happen now and then. All that is necessary is to have an arc from a grounding or an overcharged hot wire surrounded by a thick cloud of inflammable dust. If a trip of cars runs away on a dusty road and breaks the trolley or feed wires, the wrecking of the trip stirs up the air and raises dust, and if this dust is easily set on fire, the forming of an arc by the broken wires may start a flame. Laboratory experiments at Pittsburg have shown that a small white hot wire can set fire to a cloud of dust.

Many people have believed that an open flame could not ignite floating coal dust unless a shock, as from a blown-out shot, accompanied the flame. Yet floating coal dust has caught fire about bins and dusty tipples. Usually there has been little damage done because the dust was not confined, but a bad explosion in the coalgrinding room of a cement plant in New Jersey is believed to have started from the flame of an open lamp. In mines, because the space is confined in rooms and entries, there is much greater chance of the burning dust causing an explosive wave. This possibility was shown in a recent coal-mine explosion that started near the mouth of a dusty drift. This drift was an intake, so the air could not have contained any gas. A trip of cars jumped the track and knocked out some timbers, thus making a dense dust cloud that was probably fired by the flame of a trip-rider's lamp. The explosion traveled a long distance into the mine and gained strength as it traveled.

An incipient dust explosion, started by the flame of an open lamp, was seen at a mine in Yorkshire, England. The dust that ignited. was raised by a passing trip of cars. The flame followed the trip some distance and then ran back.

MOISTURE IN MINE AIR.

MOISTURE NEEDED TO RENDER COAL DUST INERT.

Experiments at the Pittsburg station and the examination of coal mines after explosions have indicated that if the coal dust in any part of a mine is so damp that every sample of it that may be gathered sticks together when pressed in the hand, there is little or no danger of a dust explosion starting in that part of the mine. To be so damp as this, the dust must hold moisture to the extent of at least onethird its own weight. The added moisture tends to keep the dust from taking fire in two ways: (1) The moist dust must be dried before it will flame, and the drying takes heat, so more heat or a larger flame is needed to ignite wet dust than dry dust; (2) moistened dust settles to the floor and packs there or sticks to the walls and roof. In case of a shock to the mine air no dust cloud can form to spread a flame.

MINE AIR IN SUMMER AND IN WINTER.

As has been stated, coal seams are generally dry. Even in the wet mines water comes in only at certain points. Such water may be considered as more or less accidental, and in most mines not artificially moistened the working places and roads are dry except in summer time.

All mining men have noticed that in summer the intake airways of mines become moist and even wet. They have noticed, too, that as the summer advances this moisture creeps in farther, until finally in most mines the dampness extends through all the workings. On the other hand, they have seen that when colder weather comes in the fall the intake airways begin to dry up, and after a while the roadways that have been damp or wet get dusty unless they are sprinkled or watered in some way.

Another fact that has been widely noticed is the relation between explosions and the time of the year. In summer the bituminous and lignite mines in this country are practically free from terrible explosions; these begin when cold weather comes. The records of State mine inspectors show that there is ground for believing that dust explosions in coal mines have been much more common in winter than in summer. Dust explosions may happen in the summer once in a while, and in districts where the air is dry, as in the Rocky Mountain coal fields, there is nearly as much likelihood of their happening in summer as in winter, unless special measures are taken to keep the dust in such condition that it will not spread a flame.

Mines are drier in winter than in summer because the outside air in winter is colder than the mine and can hold little water. When this